Calpains, a group of ubiquitous Ca2+-activated cytosolic proteases, are believed to play some role in cytoskeletal remodeling, cellular adhesion, shape change, and motility by cleaving membrane- and actin-associated cytoskeletal proteins (see, e.g., Beckerle et al., Cell 51:569-577, 1987; Yao et al., Am. J. Physiol. 265(pt. 1):C36-46, 1993; and Shuster et al., J. Cell Biol. 128:837-848, 1995). Calpains have also been implicated in the pathophysiology of cerebral and myocardial ischemia, platelet activation, NF-κB activation, Alzheimer's disease, muscular dystrophy, cataract progression, and rheumatoid arthritis.
Calpastatin is a physiological inhibitor of μ-calpain and m-calpain, which are so named because they require micromolar or millimolar concentrations of Ca2+ ions, respectively, to achieve half-maximal activity in vitro. Calpastatin has four internally repeated domains, each of which independently binds a calpain molecule in its active, Ca2+-bound conformation with high affinity (Mellgren et al., The Regulation of Calpains by Interaction with Calpastatins, and Maki et al., Structure-Function Relationship of Calpastatins, both in Intracellular Calcium-Dependent Proteolysis, Mellgren and Murachi, Eds, CRC Press, Boca Raton, Fla., 1990; and Yang et al., J. Biol. Chem. 269:18977-18984, 1994).
There is considerable interest in inhibitors of calpain (Wang et al., Trends in Pharm. Sci. 15:412-419, 1994; Mehdi, Trends in Biochem. Sci. 16:150-153, 1991).